1. Field of the Invention
The invention relates generally to the field of data processing and more specifically to disk head track centering servo systems.
2. Brief Description of the Prior Art
Disk head track centering servo systems are of two general types: dedicated servo disk and embedded servo data. In the former, a servo head reads servo data constantly from prerecorded tracks of servo data on a servo disk. In the latter, the data head reads segments of prerecorded servo data interspersed among data segments on a data track. In both cases, a position error signal is developed from the servo data which drives a servo actuator to position the servo or data head over track center. In the case of a dedicated disk, the position error signal is updated constantly. With embedded servo data, the position error signal must be held while the disk rotates over a segment of data on the data track.
Servo data is generally composed of a pattern of one type written on one track and a pattern of a second type written on an adjacent track. A head moving exactly between these tracks spans half of each servo data track and reads each half-track with equal amplitude. The head so positioned is at a data track center. However, when it is off data track center, the head will read the pattern on one servo data half-track at a higher amplitude than the pattern on the other half-track. This difference in amplitude is a measure of the distance the head is off data track center.
It is common in servo systems to peak detect the patterns from each half-track, which most commonly consist of spaced pulses of equal amplitude. The output of the peak detector is smoothed through a passive low pass filter (usually R-C) and then differentially amplified. This difference is the position error signal.
Defects in the magnetic media cause noise either in the form of high amplitude spikes or dropouts. In either case, the peak detector captures and holds a large but false value for the duration of the pulse sequence. The low pass filter holds the effect of this disturbance for a relatively long time. In the case of embedded servo segments, the false position error signal may also be held constant while the disk rottes to the next servo segment, thereby magnifying the problem.
Noise causing defects occur with increasing frequency as the number of tracks per inch increases. The move toward an increased number of tracks per inch and to the embedded servo technique thereby necessitates an improved method of demodulating the servo data.
Recent servo data demodulators have incorporated integrators to time average the signal. See e.g., U.S. Pat. Nos. 4,101,942 to Jacques, assignee Xerox Corp., 4,130,844 to Klinger, assignee Xerox Corp., and 4,208,679 to Hertrich, assignee Digital Equipment Corp. Jacques '942 and Hertrich '679 also have redesigned servo data patterns which have time-averaged self-nulling patterns when the read head is centered between the two servo half tracks. A noise spike having equal and opposite amplitude flux reversals would thus be integrated to zero.
While integrators may satisfactorily accommodate noise spikes, they are unduly susceptible to DC and offsets as recognized by Klinger '844. These must be compensated for by complicated circuitry such as that disclosed in Klinger.
It remains desirable to render demodulators for conventional servo data patterns relatively invulnerable to noise, especially in view of existing disk packs having such servo data patterns prerecorded on them.